Justice Secretary Ken Clarke has said the Government cannot "defy the law" and will have to allow at least some prisoners to vote in future.

It comes as the Commons' Political and Constitutional Reform Committee found the current blanket ban was "illegal under international law founded on the UK's treaty obligations".

MPs will debate the ban and the UK's position in Parliament on Thursday ahead of a vote on the issue.

"We have to fulfil our obligations but we are not going to give the vote to any more prisoners than was necessary to comply with the law," Mr Clarke told BBC Radio 4's Today. "What we can't do is just defy the law and pretend we are going to go wandering off."

Mr Clarke added: "The idea that we are going to give the vote to murderers and rapists and some of the more alarmist reports is complete nonsense.

"Of course, the most serious people can't possibly be given the vote - they should lose their civil rights. Probably the least serious ones will obviously get the vote and there will be a cut-off somewhere."

Meanwhile, the Archbishop of Canterbury has backed calls for prisoners to be given the right to vote. Dr Rowan Williams said prisoners' civic status should not be put in "cold storage" while they are behind bars and the UK needed to move beyond a "situation where the victimising of the prisoner by the denial of those basic civic issues is perpetuated".

The Government is currently proposing to allow the vote to all inmates serving less than four years, in response to a European Court of Human Rights ruling which could otherwise open up the floodgates to compensation claims totalling millions of pounds.

But the move - which Prime Minister David Cameron said made him feel "physically ill" - has been met by stiff opposition from some MPs, and there have been indications that the vote may be restricted to those serving a year or less.

Thursday's motion, tabled by Tory former shadow home secretary David Davis and Labour's former Justice Secretary Jack Straw, states that the decision on prisoners' votes should be one for democratically elected lawmakers and states that "no sentenced prisoner" should be granted the vote except those jailed for debt default or contempt of court.